1. Field of the Invention
The present invention relates to a thin film magnetic head. Specifically, it relates to the composition of a side shield (or side shield layer) of the thin film magnetic head provided with a pair of magnetic layers whose magnetization directions change according to an external magnetic field.
2. Description of the Related Art
Conventionally, a spin valve head is known as a head having high power and high sensitivity, used for a hard disk drive (HDD). In order to fix a magnetization direction of one ferromagnetic layer of a pair of ferromagnetic layers that are disposed on both sides of a nonmagnetic intermediate layer, an antiferromagnetic layer made of a material such as IrMn is used for the spin valve head. Since the antiferromagnetic layer has a relatively thicker film thickness, and may cause a restriction of further high recording density (narrowing a read gap), a new concept attempts to narrow a read gap. In this specification, the read gap means a gap between upper and lower shield layers.
A thin film magnetic head is disclosed in the specification of U.S. Patent Publication No. 2009/0034132. The thin film magnetic head has two free layers whose magnetization directions change according to an external magnetic field, and a nonmagnetic intermediate layer that is sandwiched by the two free layers. In this specification, a magnetoresistive (MR) element having the above-described configuration may be indicated as dual free layers (DFL). The two free layers are exchange-coupled based on RKKY (Rudermann, Kittel, Kasuya, Yoshida) interaction through the nonmagnetic intermediate layer. The two free layers are magnetized in antiparallel directions to each other under the state that a magnetic field is not applied at all. In this specification, antiparallel direction means that magnetization directions are parallel but opposed to each other. A bias magnetic layer is disposed on rear sides of the two free layers, seen from an air bearing surface (ABS), and a bias magnetic field is applied in an orthogonal direction to the ABS. Magnetization directions of the two free layers form a certain relative angle by the magnetic field generated from the bias magnetic layer. Under this state, when an external magnetic field, which is in an orthogonal direction to the ABS, is applied from a recording medium, the magnetization directions of the two free layers change, the relative angle between the magnetization directions of the two free layers changes, and an electrical resistance of a sense current changes. Therefore, the external magnetic field may be detected by using the above-described property.
Since the film configuration configured with the two free layers needs no antiferromagnetic layer, the film configuration is simplified. As a result, it becomes easier to narrow the read gap. In the document, side shield layers including NiFe are disposed on both sides of each of the two free layers in a track width direction, and it is considered to reduce an influence of a magnetic field generated from adjacent tracks. Even if an element size is the same, the side shield layers may narrow an effective track width. In order to obtain the high recording density of a hard disk drive (HDD), it is required not only to increase a recording density in a circumferential direction of a truck (a linear recording density), but also to increase an arrangement density in a diametral direction of the track (a track density). It can be said that the invention, which is disclosed in the document, effectively functions to increase both the recording and arrangement densities, and that the invention indicates one of the ideal configurations to obtain the HDD high recording density in the future.
Meanwhile, a side shield layer has a height that is almost identical to heights (depth of the free layer measured in an orthogonal direction to the surface of ABS) of the two free layers. In order to improve a response according to the external magnetic field, the height of the free layer is formed as small as possible. Therefore, the height of the side shield layer that is adjacent to the free layer is formed small, and the side shield layer is generally formed in a long and narrow shape in the track width direction. The side shield layer having such a long and narrow shape tends to be magnetized in a lengthwise direction of the long and narrow shape by a shape anisotropy effect. When the side shield layer is magnetized, not only the bias magnetic field generated from the bias magnetic layer but also a magnetic field generated from the side shield layer are applied to the free layer. Since the conventional side shield layer is mainly formed by NiFe, the side shield layer easily absorbs the bias magnetic field generated from the bias magnetic layer, and is magnetized. It is ideal that a direction of the bias magnetic field, the direction added to the free layer, is in the orthogonal direction to the ABS. However, when the side shield layer applies the magnetic field to the free layer, a similar phenomenon occurs in a case where the direction of this bias magnetic field substantially inclines in the orthogonal direction to the ABS. This phenomenon deteriorates linearity between the external magnetic field and a signal output at the time of the external magnetic field being applied, and further increases a dispersion of the output.